Add default issue template encouraging correct workflow.

The existing documentation is doing surprisingly good job. Most of the issue are reported in inbox, but there is still fraction of users who stumble across main issue tracker first.

Ideally the permissions would be setup so that guest users can only comment not create issues directly in main tracker, but I don't think that such fine grained control is available without self hosted or enterprise versions of Gitlab. It's possible to block all issue tracker interactions, but that's no good. Users should be able to comment and answer to developer questions for the issues which have been moved from inbox. This happens sufficiently rare so it's not worth investigating more complex Gitlab deployments just for this reason.